In orthopedic medicine, physicians often need to correct certain skeletal injuries or deformities with external fixator devices. These devices use pins or wires attached to the separate bone segments and an external structural frame to align, or fix, the bone segments in a way to aid in repairing the injury or correcting the deformity. Often the physician must gradually adjust the orientation of the bone segments over time, optimally with the capability to adjust the orientation along six degrees of freedom to ensure the bone segments are placed in the correct anatomic condition.
Devices and methods for treating musculoskeletal deformities are well known in the art. Although these devices vary considerably in design, they typically fall into two broad categories, circular ring and unilateral devices. The circular ring device category is exemplified by Ilizarov-type systems, which have two rings connected by linear struts with a fixed or hinged connection at each end of each strut. A device called a space frame, which has two rings connected by six linear struts having a spherical joint at each end arranged in a hexapod configuration, e.g., a Stewart frame, represents an advancement on the original Ilizarov concept.
The Ilizarov device is constructed based on the deformity that needs correcting, that is, for a specific patient and a specific deformity on that patient, hinges and struts are added to address each degree of deformity in a specific case. Ilizarov-type devices are often referred to as serial manipulators in that each adjustment relates to a single degree of deformity. This approach requires the frame to be constructed based on the deformity present, resulting in a fairly straightforward method of use but a potentially complex set of multiple and potentially endless configurations. The space frame is a device that conceptually comes in one configuration even though rings and struts can be of differing sizes. The Stewart frame-type space frame is often referred to as a parallel manipulator in that any given adjustment to any of the six struts will result in a change to all six degrees of freedom. This characteristic makes the Stewart-frame type device less intuitive to use and a computer program is often required to direct the user in making the adjustments to correct the deformity.
The unilateral device category has several devices that basically consist of a series of orthogonal planar hinges or spherical joints and, in some cases, sliders that can be locked into a particular orientation. Typically these devices can be used to fix bone segments in a particular orientation but not to gradually adjust the orientation, since the joints of the device do not have a direct adjustment device associated with each hinge or slider. Instead, the joints of the device must be loosened and then grossly manipulated on the device as a whole. Like the Ilizarov-type circular ring fixators, these devices often need to be constructed or mounted in a particular orientation depending on the characterization of the deformity. This requirement complicates their use and also necessitates multiple configurations to address the range of deformities that physicians typically encounter.
An innovative unilateral fixator, described in U.S. Non-Provisional patent application Ser. No. 10/664,769, entitled Unilateral Fixator, incorporated herein by reference, provides a six-degree-of-freedom manipulator in the form of an open kinematic chain, similar to many industrial robot arms in use today. At the start of the kinematic chain is the connection to what would be the reference bone segment, otherwise known as the ground. At the end of the kinematic chain is the connection to what would be the moving bone segment. Each of the links in the kinematic chain is made up of a rigid structural member with the connection between links being the joints.
The positioning and adjusting of a fixator, such as a six-degrees-of-freedom unilateral fixator described in U.S. Non-Provisional patent application Ser. No. 10/664,769 allows a physician to correct a deformity. The starting point for determining the positioning and adjusting of the fixator includes x-rays that depict a given deformity. Typically, measurements such as the axial rotation, the anterior-posterior (AP) rotation, the lateral rotation, the pin offset, and bone length, are used to position and adjust a fixator. A physician may make these measurements directly on x-rays of a deformity. Although this technique is adequate for characterizing a deformity, these manual measurements do require increased work for a physician and may introduce error. An improved method would take advantage of digital imaging to reduce or eliminate the need for manual measurements.
What is needed is a method of deformity characterization and description of the relative mounting orientation of a deformity-correcting fixator device relative to the deformity such that a mathematical transform can be obtained is needed. Optimally, the method would minimize measurements taken by a physician. Also, the method may be adaptable to characterizing any type of tissue configuration captured on an x-ray.